Add support for RubyGems >= 2.0.0
RubyGems 2.0.0preview2 removed --no-rdoc and --no-ri options for the install command. (https://github.com/rubygems/rubygems/blob/master/History.txt)
RubyGems 2.0.0preview2 removed --no-rdoc and --no-ri options for the install command. (https://github.com/rubygems/rubygems/blob/master/History.txt)